This invention relates to a coin-shaped lithium battery constituted by a battery can containing therein an anode pellet composed of lithium or lithium alloy and a cathode pellet which are arranged to face each other with a separator provided between them.
In the coin-shaped lithium battery constituted by a battery can containing therein an anode pellet composed of lithium or lithium alloy and a cathode pellet which are arranged to face each other with a separator provided between them, such as a coin-shaped lithium manganese battery, the anode material, that is, the anode pellet composed of lithium or lithium alloy decreases in volume, and the cathode material, that is, the cathode pellet increases in volume as the battery discharges. The decrease in height of the anode pellet and the increase in height of the cathode pellet are not equal, reducing the total height of the anode pellet and the cathode pellet. Consequently, contact of the battery can, the anode pellet and the cathode pellet with one another, that is, contact between anode and cathode cannot be maintained satisfactorily in the latter half of discharge, and hence the unsatisfactory contact causes a significant increase in internal pressure and discharge failure.
In a conventional coin-shaped lithium manganese battery constituted by a battery can 44 containing therein parallel disc-shaped anode pellet 41 and cathode pellet 42 facing each other with a separator 43 between them, an anode cap 45 and a cathode can 46 forming the battery can 44 are caulked with an insulation sealing gasket 47 provided between them, and the anode cap 45 is elastically deformed, as shown in FIG. 1. In the coin-shaped lithium manganese battery of this structure, hereinafter referred to as Conventional Example 1, the decrease in height of the active material in the course of discharge causes the anode cap 45 to elastically deform, so that satisfactory contact between anode and cathode is maintained.
In another conventional coin-shaped lithium manganese battery constituted by a battery can 54 containing therein an anode pellet 51 and a cathode pellet 52 facing each other with a separator 53 between them, the cathode pellet 52 is fitted into a cathode ring 55, as shown in FIG. 2. In the coin-shaped lithium manganese battery of this structure, hereinafter referred to as Conventional Example 2, the cathode ring 55 limits the change in volume of the cathode pellet 52 in the course of discharge, to the vertical direction. Thus, the reduction in the total height of the anode pellet 51 and the cathode pellet 52 during discharge is prevented.
In addition, another coin-shaped lithium manganese battery, hereinafter referred to Conventional Example 3, has been proposed. This coin-shaped lithium manganese battery of Conventional Example 3 has a thinner battery can 64 which contains therein an anode pellet 61 of greater diameter than that of Conventional Example 1 and a cathode pellet 62 facing each other with a separator 63 between them, as shown in FIG. 3.
With above-described conventional coin-shaped lithium manganese batteries, internal resistance R1 prior to discharge and internal resistance R2 after discharge of 80% of nominal capacity were measured. The results are shown in Table 1.
TABLE 1 ______________________________________ Internal Internal Nominal Discharge Resistance Resistance Battery Capacity Capacity R1 R2 ______________________________________ Conventional 190 mAh 191 mAh 8 .OMEGA. 63 .OMEGA. Example 1 Conventional 190 mAh 193 mAh 8 .OMEGA. 20 .OMEGA. Example 2 Conventional 220 mAh 228 mAh 7 .OMEGA. 101 .OMEGA. Example 3 ______________________________________
The coin-shaped lithium manganese battery of Conventional Example 1 has a problem such that internal resistance increases in the latter half of discharge, as shown in Table 1. This increase in internal resistance in the latter half of discharge indicates that the battery in practical use has its discharge voltage reduced when heavy load which deteriorates battery properties particularly at low temperatures is imposed on the battery. Consequently, equipments using this battery may not operate normally.
The coin-shaped lithium manganese battery of Conventional Example 2 requires the cathode ring 55, and thus has problems of increase in the number of component parts and hence increase in costs.
The coin-shaped lithium manganese battery of Conventional Example 3, though having a greater capacity than in Conventional Example 1, has problems of larger increase in internal resistance in the latter half of discharge and deterioration of performance by heavy load particularly at low temperatures.